The traffic on optical-fiber communication networks that serve the Internet-oriented society of recent years continues to increase year after year, and now the bit rate of commercial wavelength-division-multiplexing (WDM) communication devices has reached 40 Gb/s. IEEE 802.3 is standardizing a capacity of 100 Gb/s as a next-generation fast Ethernet. ITU-T SG15 has started studying accommodation schemes for optical transport networks (OTN) of 100 GbE, and is actively researching and developing optical fiber transmission using fast signal light on the order of 100 Gb/s.
Generally, such a super-fast signal is generated by optical time-division-multiplexing (OTDM) short pulses of about several picoseconds. Since electrical signal processing cannot keep up with such a super-fast signal, all-optical signal processing, in which an optical signal is processed as it is without being converted into an electrical signal, is under research. The speed of the electrical signal processing has improved recently, however, the limit is anticipated to be about 100 GHz.
As one elementary technology for all-optical signal processing, an optical switch that operates at very high speed is important. For example, to monitor the quality of the super-fast signal, the symbol error rate has to be measured and the optical waveform has to be observed. In this case, since current technology depends on electrical signal processing, the bit rate has to be reduced by time-division-demultiplexing the super-fast signal to a speed that the electrical signal processing can keep pace with.
An ultra high-speed optical switch is not only used for such relatively-simple time-division-demultiplexing, but also is expected to be used as an all-optical 2R regenerator or 3R regenerator that restore signal quality that has been deteriorated by noise of the optical fiber transmission path, the optical amplifier, etc. The 2R regeneration means reamplification and reshaping, and the 3R regeneration means reamplification, reshaping, and retiming.
Such an ultra high-speed optical switch is not feasible yet, however, several schemes have been suggested. Here, the ultra high-speed optical switch means an optical AND gate. Particularly, an optical fiber switch can operate at very high speed since the optical fiber switch uses a nonlinear optical effect of an optical fiber responding within about femtosecond, and has a low insertion loss and high efficiency. Thus, the optical fiber switch is a promising optical switch for practical use.
As an optical fiber switch, various kinds of devices are known such as an optical Kerr switch using cross phase modulation (XPM), a nonlinear optical loop mirror (NOLM), a phase conjugator using four wave mixing (FWM), a wavelength converter, a parametric amplifier, and a parametric amplification switch, etc.
In an optical switch using a nonlinear optical effect, an optical pulse that can achieve a higher peak power than continuous light and can cause the nonlinear optical effect efficiently is used as control light. Not limited to the optical AND gate, nonlinear optical effects are used variously in the field of all-optical signal processing, such as a wavelength converter using self phase modulation (SPM), pulse compression, etc.
Related to the optical signal processing, a technology for generating a flat-top optical pulse or an optical pulse of quadratic shape has been disclosed. For example, a technology for generating an optical pulse using the birefringence of a polarization maintaining fiber (PMF) has been disclosed (PMF method; see, for example, Schubert, C., et al, “160 Gbit/s wavelength converter with 3R-regenerating capability,” Electronics Letters, Volume 38, Issue 16, 1 Aug. 2002, Pages 903-904).
An optical signal processing device that generates an optical pulse by PMF method is configured with, for example, a PMF and a polarizer. Light is input to the PMF at an orientation at which the power of the optical pulses is equally distributed to the fast axis (x axis) and the slow axis (y axis) having different refractive indices, that is, at an orientation shifted from the x axis by 45 degrees. In the PMF, the propagation speeds of the optical pulses along the axes are different.
Thus, optical pulses that have propagated through the PMF exhibit differences in arrival time (differential group delay). A pulse wider than the input optical pulse can be obtained by extracting a polarization component from an optical pulse that has propagated through the PMF by the polarizer at the orientation shifted from the x axis by 45 degrees. A flat-top optical pulse can be obtained by adjusting the width of the optical pulse and the differential group delay of the PMF.
An optical signal processing device that generates an optical pulse by slicing the optical spectrum by a fiber bragg grating (FBG) has been disclosed (FBG method; see, for example, Petropoulos, P., et al, “Rectangular Pulse Generation Based on Pulse Reshaping Using a Superstructured Fiber Bragg Grating,” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 19, No. 5, MAY 2001). The FBG method is a method of reshaping the optical pulse by reshaping the optical spectrum that is Fourier transform components of the optical pulse.
A flat-top optical pulse can be obtained by designing the shape of the reflection band of the FBG to be the shape resulting from Fourier transform of the flat-top optical pulse and by slicing the optical spectrum by the FBG. A parabolic optical pulse of quadratic shape can be obtained also by designing the reflection band of the FBG to be a quadratic (parabola) shape.
Related to optical signal processing, a scheme called optical Fourier transform has been suggested as a measure of restoring the quality of signal light (see, for example, Hirooka, Toshihiko, et al, “A New Adaptive Equalization Scheme for a 160-Gb/s Transmitted Signal Using Time-Domain Optical Fourier Transformation,” IEEE, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, No. 10, OCTOBER 2004, Pages 2371-2373). The optical Fourier transform is a method of converting, by a phase modulator and a dispersion medium, the waveform of an optical pulse to the optical spectrum that is Fourier transform components of the optical pulse and vice versa. Even if the waveform of the optical pulse is distorted due to dispersion, dispersion slope, or polarization mode dispersion, etc., the waveform of the optical pulse can be restored by the optical Fourier transform as long as the shape of the optical spectrum is maintained.
The optical Fourier transform achieves efficient regeneration by multiplying phase modulation for quadratic shape in synchronization with the optical pulse. An optical signal processing device that performs a conventional optical Fourier transform is implemented by a phase modulator using, for example, LiNbO3. Further, another technology for generating an optical pulse of quadratic shape has been disclosed (see, for example, Parmigiani, F., et al, “Pulse Retiming Based on XPM Using Parabolic Pulses Formed in a Fiber Bragg Grating,” IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, No. 7, APRIL 1, 2006).
However, the characteristics of the signal light output from a conventional optical switch using the nonlinear optical effect described above strongly depend on the characteristics of the control light input to the optical switch. Thus, if phase noise is included in the signal light or the control light, intensity noise is generated in the signal light output from the optical switch. This problem is explained in detail below.
SPM and XPM are phase modulation (frequency chirp) generated in nonlinear medium such as an optical fiber. SPM is frequency chirp induced by the light wave of the signal light itself, while XPM is frequency chirp induced by the light wave of other wave(s) (control light). The frequency chirp induced by SPM is represented by a time differential dφSPM/dt of the phase shift φSPM induced by SPM.
The frequency chirp induced by XPM is represented by the time differential dφXPM/dt of the phase shift φXPM induced by XPM. The frequency chirp dφSPM/dt induced by SPM and the frequency chirp dφXPM/dt induced by XPM are represented by the equations (1) and (2) below, where the length of the highly nonlinear fiber is L (km), the optical power of the electric field is P1, P2 (W), and the nonlinear coefficient of the highly nonlinear fiber is γ(W−1km−1).
                                          ⅆ                          φ              SPM                                            ⅆ            t                          =                  γ          ×          L          ×                                    ⅆ                              P                1                                                    ⅆ              t                                                          (        1        )                                                      ⅆ                          φ              XPM                                            ⅆ            t                          =                  2          ×          γ          ×          L          ×                                    ⅆ                              P                2                                                    ⅆ              t                                                          (        2        )            
The nonlinear coefficient γ of equations (1) and (2) above is represented by the equation (3) below, where the nonlinear refractive index of the highly nonlinear fiber is n2, the effective core cross-sectional area of the highly nonlinear fiber is Aeff, and the wavelength of the signal light is λ.
                    γ        =                              2            ⁢                                                  ⁢            π            ⁢                                                  ⁢                          n              2                                            λ            ⁢                                                  ⁢                          A              eff                                                          (        3        )            
As indicated by equations (1) and (2) above, the frequency chirp induced by SPM or XPM is proportional to the time differential dPn/dt of the optical power (n=1 or 2). Thus, the characteristics of the signal light output from the optical switch using a nonlinear optical effect strongly depend on the characteristics of the control light input to the optical switch. For example, when the optical pulse has a quadratic shape, linear frequency chirp proportional to time can be provided to the signal light output from the optical switch.
FWM is a phenomenon that, when a control light λc having the same wavelength as the zero-dispersion wavelength λ0 of the optical fiber and a signal light λs having a different wavelength from the control light are both input to the optical fiber, a third light (idler) is generated. In this phenomenon, a gain of the same power as the idler is generated for the signal light. This is called parametric amplification. The generation efficiency η of FWM is represented by the equation (4) below, where the loss of the nonlinear fiber is α, and the power of the control light at the input to the optical fiber is Pc.η=exp(−α×z)×(⊖×Pc×L(z))2  (4)
L(z) of equation (4) above is represented by the equation (5) cited below, where the length of the nonlinear optical fiber is z.
                              L          ⁡                      (            z            )                          =                              1            -                          exp              ⁡                              (                                                      -                    α                                    ⁢                                                                          ⁢                  z                                )                                              α                                    (        5        )            
In equation (5) above, L=z when the loss of the optical fiber can be disregarded (α=0). Thus, in equation (4) above, the generation efficiency η of FWM is proportional to the square of the optical power Pc when γ×Pc×z is sufficiently large and the loss of the optical fiber can be disregarded. As a result, when the light wave is an optical pulse, the generation efficiency of FWM differs according to the position of the waveform of the optical pulse. Thus, in the optical switch using a nonlinear optical effect, the generated nonlinear optical effect depends on the shape of the optical pulse in principle.
FIG. 18 is a diagram for explaining the operation of a conventional optical switch. In FIG. 18, signal light 1810 represents signal light input to an optical switch 1830. An optical pulse 1820 represents an optical pulse input to the optical switch 1830 as control light. The optical switch 1830 is an optical switch using a nonlinear optical effect described above. Here, an example is described in which the signal light 1810 includes phase noise.
The nonlinear optical effect is generated only when the signal light 1810 and the control light 1820 that are input to the optical switch 1830 functioning as an optical AND gate are overlapped with each other temporally. When the signal light 1810 includes phase noise, since the optical pulse 1820 has peaks, the timings of the signal light 1810 and the peaks of the optical pulse 1820 do not match, thereby causing intensity noise 1850 on an output signal 1840 of the optical switch 1830.
It is known that phase noise is not only caused by the light source itself, but also is accumulated by propagation through optical fibers and/or repeated reamplification by optical amplifiers. Further, in the optical fiber housed in the ultra high-speed optical switch, loss in temporal overlap (walk off) is caused between the signal light 1810 and the control light 1820 due to wavelength dispersion, which is one of the causes of a reduced efficiency of the nonlinear optical effect and a limited operation band.
To cope with the above problem, the generation of intensity noise in the output signal 1840 can be prevented by reshaping the signal light 1810 or the control light 1820 input to the optical switch 1830 to have a flat-top shape. For example, a pulse waveform close to the flat-top shape can be obtained by changing the bias point of the input electrical signal by a light source with an intensity modulator. However, the pulse width that can be achieved by this method is on the order of several tens of picoseconds, and thus an optical pulse applicable to 100 Gb/s or more cannot be generated.
The method according to Schubert, C., et al, “160 Gbit/s wavelength converter with 3R-regenerating capability” strongly depends on the angle of input to the PMF, and the adjustment and the retention of settings are difficult. Further, in principle, a large insertion loss of 3 dB or more is generated. Furthermore, when an optical pulse including intensity noise is reshaped, a flat-top optical pulse inevitably including intensity noise is generated.
The method according to Petropoulos, P., et al, “Rectangular Pulse Generation Based on Pulse Reshaping Using a Superstructured Fiber Bragg Grating” has a limitation on the input/output optical pulse width. Further, in principle, the phase of the optical pulse is affected. Furthermore, when an optical pulse including intensity noise is reshaped, a flat-top optical pulse inevitably including intensity noise is generated. In addition, the methods according to Petropoulos, P., et al, “Rectangular Pulse Generation Based on Pulse Reshaping Using a Superstructured Fiber Bragg Grating” and Parmigiani, F., et al, “Pulse Retiming Based on XPM Using Parabolic Pulses Formed in a Fiber Bragg Grating” use an FBG corresponding to each wavelength, and thus cannot adapt to an optical pulse of an arbitral wavelength.
The method according to Hirooka, Toshihiko, et al, “A New Adaptive Equalization Scheme for a 160-Gb/s Transmitted Signal Using Time-Domain Optical Fourier Transformation” performs phase modulation by electrical signal processing. Thus, the bit rate of the signal light has to be reduced to about 10 Gb/s to perform Fourier transform on super-fast signal light of, for example, 100 Gb/s or more. As a result, the bit rate has to be changed again when the regenerated signal light is transmitted.
An optical pulse generating device and an optical signal processing device disclosed herein solve the above problems, and aim to generate a fast and low-noise optical pulse.